Evidence of a merger of binary white dwarfs:: The case of GD 362

GD 362 is a massive white dwarf with a spectrum suggesting a H- rich atmosphere that also shows very high abundances of Ca, Mg, Fe, and other metals. However, for pure H- atmospheres, the diffusion timescales are so short that very extreme assumptions have to be made to account for the observed abundances of metals. The most favored hypothesis is that the metals are accreted from either a dusty disk or an asteroid belt. Here we propose that the envelope of GD 362 is dominated by He, which at these effective temperatures is almost completely invisible in the spectrum. This assumption strongly alleviates the problem, since the diffusion timescales are much larger for He- dominated atmospheres. We also propose that the He- dominated atmosphere of GD 362 is likely to be the result of the merger of a binary white dwarf, a very rare event in our Galaxy, since the expected Galactic rate is similar to 10(-2) yr(-1).